The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses (“PALs”), are used for the treatment of presbyopia. PALs have at least one progressive surface that provides far, intermediate, and near vision in a gradual, continuous progression of vertically increasing dioptric power from far to near focus or top to bottom of the lens.
Typical PALs have a 12 to 14 mm in length channel between the fitting point and the point along the prime meridian of the lens at which the power reaches 85% of the lens' add power. Although such a channel helps provide more gradual power changes and lower levels of unwanted astigmatism, the near viewing zone is often cut-off when lenses of these designs are mounted into standard frames, especially frames of smaller diameters.
Some PAL lenses have been designed with shortened channels in the 9 mm to 12 mm range, but these PALs typically have much higher levels of unwanted astigmatism and a shortened intermediate viewing zone. The result of this can be an intermediate viewing zone that is unusable.